Not plastic. Although all stainless steel sippy cups have plastic parts, your child's drink is in contact with the plastic for a very short period of time (although your child's mouth is on the plastic part while drinking). No leaching of chemicals into your beverage as it sits in the container. For me, this is the big selling point.

Keep beverages cold longer. This is especially awesome for milk. Stainless steel sippy cups have saved us from throwing away a lot of spoiled milk. If you really want your sippy to keep your milk cold, you can get an insulated one, although they cost a bit more.

More durable. You have to replace the plastic valve every 3-6 months, but the steel part should last pretty much forever. No worries about chemicals leaching as the product ages either.

Steel body is dishwasher safe (usually).

CONS

Have plastic parts that are more likely to break (since they get impacted against stainless steel rather than plastic)

Klean Kanteen Sippy Cups
We love our Klean Kanteen sippy cups. The Avent spout is easy even for young babies to use. It's heavy, but my babies have been able to wield them independently by about age one. One of the big bonuses is that the Klean Kanteen sippy cups can eventually be used like a regular small water bottle by swapping out the sippy spout for a loop cap, sports cap, or other regular lid, which means it can be used pretty much forever as a little water bottle.

The plastic ring around the neck of the Klean Kanteen has broken several times when my toddlers tossed them from the high chair onto the tile floor, which is a bummer. It doesn't break every time it hits the floor, but if your child tosses it multiple times per day onto a tile floor, it will eventually break. I solved this problem for a time by only letting my baby use it while sitting on the floor. Klean Kanteen does have excellent customer service, and will replace your broken caps for free. I also find it leaks a bit out of the little air hole at the top, especially if it is quite full. This is kind of annoying, but not a huge deal. Dishwasher safe, but I only hand wash the plastic parts.

The Safe Sippy
The Safe Sippy is reasonably easy to drink from and hold. My daughter used one since around 9-months to a year. It also now comes with a straw attachment, which my kids would probably like since they are obsessed with straws. It also now comes with a little cover for the spout. It comes with removable handles (these never help my kids and I always remove them). It also comes with its own silicone sleeve. The plastic lid/spout seems to be made of plastic with a little more give than Klean Kanteen's plastic parts, so that it seems less likely to break when it hits the floor. Mine have never broken, but we use these less than the Klean Kanteen sippy cups, for reasons I can't fully explain.

The Safe Sippy replacement valves are generally a bit cheaper than the Klean Kanteen valves (which are actually Avent valves). I also find the spout/valve quicker and easier to clean than the Klean Kanteen Avent spout. However, the Safe Sippy does leak quite a bit. In fact, they state that it's not really to designed to be leak-proof, but rather to prevent major spills. If it is full, and lying on its side, it will dribble out liquid. Also, it sometimes makes a popping wheezing noise after use. You probably won't notice unless it's sitting on your bedside table as you try to go to sleep (this is how I discovered the funny noise the valve sometimes makes). Dishwasher safe, but I only hand wash the plastic parts. You don't need to remove the sleeve to put through the dishwasher, and I never have.

Eco VesselThe Eco Vessel sippy cup was unwieldy to hold (it's really fat). It does come with handles, but I never find those helpful. It uses the Avent valves, like Klean Kanteen. The plastic ring around the neck broke about the second time my daughter dropped it. The steel body is not dishwasher safe. I am sorry I bought it. (I bought it because it was cheaper.)

[P.S. I can't remember which sippy cups come with the little plastic cover that goes over the spout, because I lose these within 5 minutes of opening the package, so check the product page for that information.]

The first sippy our little guy had was the Kleen Kanteen. The plastic ring broke and the whole container is so dented from him throwing it around. I am going to keep it for when we go camping but in the mean time I have had to replace it with another brand until he gets out of this throwing phase.

If the ones that you tried leak, you should try the Ecococoon drink bottles. They are completely leak-proof in my experience, and there are three different sizes - the small ones aren't that heavy. They work so well (both not leaking and keeping the drinks cold ALL DAY) that after trying one, I got three more for the rest of my kids. Expensive, but I don't expect to have to buy another sippy cup ever again! http://leave-happier.hostedbywebstore.com/